Wireless devices use high frequency, generally referred to as Radio Frequency (RF), signals to transmit information through free space. For example, cell phones use amplified RF to transmit voice data to base stations, which allow signals to be relayed to communications networks. Other existing wireless communication devices include Bluetooth, HomeRF and WLAN. Note that the term “Radio Frequencies” is used herein to refer to any frequencies suitable for wireless communication, including Ultra High Frequency (UHF), Very High Frequency (VHF), Microwave Frequency, etc.
In a conventional wireless device, the power amplifier generally consumes most of the power of the overall wireless system. For systems that run on batteries, a power amplifier with a low efficiency (Pout/Psupply) results in a reduced communication time for a given battery life. A decrease in efficiency of the power amplifier typically results in increased power usage and heat removal requirements, which may increase the equipment and operating costs of the overall system.
For this reason, much effort has been expended on increasing the efficiency of RF power amplifiers.
A class AB amplifier, that is an amplifier with amplifying device(s) biased in class AB is typically considered the standard against which other amplifier architectures are compared in terms of gain and efficiency.
One type of amplifier that may increase power amplifier efficiency is a Doherty-type power amplifier. A common Doherty-type power amplifier design includes a main amplifier and an auxiliary amplifier. The main amplifier is operated to maintain optimal efficiency up to a certain power level and allows the auxiliary amplifier to operate above that level. When the power amplifier is operated at a high output power level, the gain of the main amplifier will be heavily compressed such that non-linearities are introduced into the amplified signal.
In a conventional Doherty-type amplifier, a signal preparation unit, often implemented with a simple power splitting structure, is used to divide an amplifier input signal along main and auxiliary amplification paths to the main and auxiliary amplifiers, respectively, for amplification.
FIG. 1 is a block diagram of a conventional Doherty-type amplification unit 100. As shown in FIG. 1, Doherty-type amplification unit 100 comprises an input signal line 106, a main amplifier 102, an auxiliary amplifier 104, a signal preparation unit 110, a main amplifier impedance transformer 112, and an output signal line 108. An input signal is passed into input signal line 106 and into signal preparation unit 110. Signal preparation unit 110 transmits the input signal from input signal line 106 into main amplifier 102, and signal preparation unit 110 phase shifts the input signal from input signal line 106 and transmits the phase shifted signal to auxiliary amplifier 104. A combining structure 114, which includes the main amplifier impedance transformer 112 in the main amplifier signal path at the output of the main amplifier 102, receives output from the main amplifier 102 and output from the auxiliary amplifier 104 and combines them to form an output signal that is transmitted to signal output line 108.
In some conventional Doherty-type amplifiers, the phase shift introduced by signal preparation unit 110 is corrected in main amplifier impedance transformer 112, so that the signal exiting main amplifier impedance transformer 112 is in phase with the signal that exits auxiliary amplifier 104.
In many conventional Doherty-type amplifiers, a matching structure (not shown in FIG. 1) is used to match the output impedance of the Main and Auxiliary amplifiers to the input impedance of the device that is being driven by the output of the Doherty-type amplifier, typically an antenna structure.
Although Doherty-type amplifiers are often superior in terms of efficiency over Class AB amplifiers, they are often plagued by reduced gain levels. Typical Doherty-type amplifier designs (Classical, Asymmetrical and Enhanced asymmetrical) generally incorporate a power divider that either separates the input signal power equally, or in a pre-set ratio, to the Main and Auxiliary amplifier paths. However, the portion of the input signal that is directed to the Auxiliary amplifier path is typically used to self-bias the Auxiliary amplifier in back-off and therefore does not develop measurable power at the output of the Auxiliary amplifier since the Auxiliary amplifier is typically off during back-off operation thereby reducing the overall gain of the conventional Doherty-type amplifier. In other words, the portion of the input signal that is directed to the Auxiliary amplifier in a conventional Doherty-type amplifier is not amplified during back-off operation thereby lowering the overall gain.
FIG. 2 is a plot of gain and efficiency versus output power for a conventional AB amplifier at 200, 202 respectively and a plot of gain and efficiency versus output power for a conventional Doherty-type amplifier at 206, 204 respectively. It can be seen from FIG. 2 that efficiency 200 of the class AB amplifier is much poorer than the efficiency 204 of the conventional Doherty-type amplifier. However, the gain 202 of the class AB amplifier is almost 4 dB higher than the gain 206 of the conventional Doherty-type amplifier.
In order to increase the gain of a conventional Doherty-type amplifiers, larger drive circuitry, i.e., larger Main and Auxiliary path drivers (or amplifiers prior to the Signal Preparation Unit, 110) that consume more power, are typically used, which quickly reduces the efficiency improvements achieved through use of the Doherty architecture.
In common Doherty-type amplifiers, the main and auxiliary amplifiers are composed of the same type of amplifiers with the same power amplification rating. These Doherty-type amplifiers develop an efficiency peak 6 dB back of full power which in theory will be equal in magnitude to the maximum efficiency of the system.
In asymmetrical Doherty-type amplifiers, the main and auxiliary amplifiers are implemented with devices of unequal size in terms of power. Accordingly, traditional Doherty characteristics, specifically efficiency versus output power, can be modified in order to adjust the location of the peak efficiency in back-off.
A further advance over asymmetrical Doherty-type amplifiers is provided in an enhanced asymmetrical Doherty-type amplifier, as described in U.S. Patent Application Publication No. US 2008/0088369, published Apr. 17, 2008, which is assigned to the Assignee of this application, and is hereby incorporated by reference in its entirety. An enhanced asymmetrical Doherty-type amplifier utilizes an asymmetrical Doherty-type amplifier structure with Main and Auxiliary amplifier devices of different semiconductor technologies to take advantage of the performance characteristics, for example, linearity and power handling characteristics, of the different semiconductor technologies.
With enhanced asymmetrical Doherty-type amplifiers, relatively high efficiency can be tailored to a given output power transfer function with different sizes, in terms of power, for the Main and Auxiliary amplifiers, as well as the use of different semiconductor technologies to implement the Main and Auxiliary amplifiers. In some implementations, various classes of device biasing are used to bias the devices used to implement the Main and Auxiliary amplifiers. Examples of potential device biasing classes include, but are not limited to, Class A, AB, B, C, D and H.
An amplifier device biased in class A conducts current at all times, Class B amplifiers are designed to amplify half of an input wave signal, and Class AB is intended to refer to the Class of amplifier which combines the Class A and Class B amplifier. As a result of the Class B properties, Class AB amplifiers are operated in a non-linear region that is only linear over half the wave form. Class C amplifiers are biased well beyond cut-off, so that current, and consequently the input signal, is amplified less than one half the duration of any given period. The Class C design provides higher power-efficiency than Class B operation but with the penalty of higher input-to-output nonlinearity.
Furthermore, these types of amplifiers are high in memory, i.e., previous operating states effect the current state, and therefore are difficult to “correct” or “linearize”, due to their complex gain and phase profiles.
Asymmetrical Doherty-type amplifiers are even more difficult to linearize as the typically smaller Main amplifier is pushed deeper into compression before the Auxiliary amplifier turns on to handle the higher input power levels.
Enhanced asymmetrical Doherty-type amplifiers are again more difficult to linearize due to the asymmetrical transfer functions, specifically in terms of gain and phase, which the mixed semiconductor devices introduce.
Conventional pre-distortion algorithms typically cannot provide sufficient correction of asymmetrical and/or enhanced asymmetrical Doherty-type amplifiers such that they comply with transmission standards such as CDMA, UMTS, HSPA, OFDM, WiMAX, LTE and multicarrier GSM. Consequently, unlinearized enhanced asymmetrical Doherty amplifiers are generally unsuitable for linear modulation systems.
In addition to the gain and linearization issues, conventional Doherty-type amplifiers typically only operate over a narrow frequency band, which is typically limited by the Doherty combining and matching structures utilized to combine the outputs of the main and auxiliary amplifier paths and match the outputs of the main and auxiliary amplifiers to a load impedance. Accordingly, conventional Doherty-type amplifiers are typically limited to narrow band applications.